Recent studies of the early mRNAs of Simian virus 40 (SV40) have demonstrated the existence of a small RNA, approximately 70 nucleotides, which can anneal to the early mRNAs and to the late (L)-DNA strand at SV40 map position 0.21. This SV40-associated small RNA (SAS-RNA) arises in relatively large quantities late in the lytic cycle. Although the SAS-RNA has a precise sequence, its origin (host or viral), control of synthesis and function are unknown. The temporal induction and specific sequence of SAS-RNA, as well as its interesting region of homology on the viral genome, suggest a possible role in the control of SV40 gene expression. A goal of this research proposal is to thoroughly characterize the SAS-RNA, determine its source, the control of its synthesis, and its function in the viral cycle, during permissive, abortive and transforming infections. In separate experiments, examination of the two SV40 early mRNAs (the large T- and small t-antigen mRNAs) has shown that change in the incubation temperature of wild type SV40 lytically infected or transformed cells will alter the cytoplasmic ratio of the two mRNAs. These mRNAs are very similar, being encoded within the same DNA sequence but differing in the position of their splice sites. Thus the temperature effects on the early mRNA ratios indicate a level of gene expression control which allows preferential synthesis, formation, or stabilization of one early mRNA or the other. This apparently depends on the thermodynamic environment of the cell. The goal of the experiments described for this section is to characterize the factors involved in the determination of the ratio of early mRNAs. This characterization will allow the studies to be extended to analysis of the molecular mechanism of this level of gene expression control.